This invention relates to active acoustic devices and more particularly to panel members for which acoustic action or performance relies on beneficial distribution of resonant modes of bending wave action in such a panel member and related surface vibration; and to methods of making or improving such active acoustic devices.
It is convenient herein to use the term xe2x80x9cdistributed modexe2x80x9d for such acoustic devices, including acoustic radiators or loudspeakers; and for the term xe2x80x9cpanel-formxe2x80x9d to be taken as inferring such distributed mode action in a panel member unless the context does not permit.
In or as panel-form loudspeakers, such panel members operate as distributed mode acoustic radiators relying on bending wave action induced by input means applying mechanical action to the panel member; and resulting excitation of resonant modes of bending wave action causing surface vibration for acoustic output by coupling to ambient fluid, typically air. Revelatory teaching regarding such acoustic radiators (amongst a wider class of active and passive distributed mode acoustic devices) is given in our patent application Ser. No. 08/707,012; and various of our later patent applications concern useful additions and developments.
Hitherto, transducer locations have been considered as viably and optimally effective at locations in-board of the panel member to a substantial extent towards but offset from its centre, at least for panels that are substantially isotropic as to bending stiffness and exhibit effectively substantially constant axial anisotropy of bending stiffness(es). Aforementioned patent application Ser. No. 08/707,012 gives specific guidance in terms of optimal proportionate co-ordinates for such in-board transducer locations, including alternatives; and preference for different particular co-ordinate combinations when using two or more transducers.
Various advantageous applications peculiar to the panel-form of acoustic devices have been foreshadowed, including carrying acoustically non-intrusive surfacing sheets or layers. For example, physically merging or incorporating into trim or cladding is feasible, including as visually virtually indistinguishable. Also, functional combination is feasible with other purposes, such as display, including pictures, posters, write-on/erase boards, projection screens, etc. The capability effectively to hide in-board transducers from view is enough for many applications. However, there are potential practical applications where it could be useful to leave larger, particularly central, panel regions unobstructed even by hideable transducers. For example, for video or other see-through display use, pursuit of translucence, even transparency, of panel members is not worthwhile with such in-board intrusions of transducers, though a panel-form acoustic device would be highly attractive if it could afford large medial areas of unobstructed visibility.
According to one device aspect of this invention, there is provided a panel-form acoustic device comprising a distributed mode acoustic panel member with transducers located at a marginal position, the arrangement being such as to result in acoustically acceptable effective distribution and excitement of resonant mode vibration. Existence of suitable such marginal positions is established herein as locations for transducer, along with valuable teaching as to judicious selection or improvement of one or more such locations. Such judicious selection may advantageously be by or as would result from investigation of an acoustic radiator device or loudspeaker relative to satisfactorily introducing vibrational energy into the panel member, say conveniently by assessing parameters of acoustic output from the panel member concerned when excited at marginal positions or locations. At least best results also apply to microphones.
From the relevant background teaching as of the time of this invention, availability of successful such marginal locations is, to say the least, unexpected. Indeed, main closest prior art cited against patent application Ser. No. 08/707,012, is the start-point for its invention and revelatory teaching, namely WO92/03024 from which progress was made particularly in terms of departing from in-corner excitation thereof. Such progress involved appreciating that distributed resonant mode bending wave action as required for viable acoustic performance results in high vibrational activity at panel corners; as is also a factor for panel edges generally. At least intuitively, and as greatly reinforced by practical success with somewhat off-centre but very much in-board transducer locations, such high vibrational activity compounds strongly with panel margins self-evidently affording limited access, thus likely available effect upon, panel member material as a whole; this compounding combination contributing to previously perceived non-viability of edge excitation.
For application of this invention, a suitable acoustic panel member, or at least region thereof, may be transparent or translucent. Typical panel members may be generally polygonal, often substantially rectangular. Plural transducers may be at or near different edges, at least for substantially rectangular panel members. The or each transducer may be piezo-electric, electrostatic or electro-mechanical. The or each transducer may be arranged to launch compression waves into the panel edge, and/or to deflect the panel edge laterally to launch transverse bending waves along a panel edge, and/or to apply torsion across a panel corner, and/or to produce linear deflection of a local region of the panel.
Assessment of acoustic output from panel members may be relative to suitable criteria for acoustic output include as to amount of power output thus efficiency in converting input mechanical vibration (automatically also customary causative electrical drive) into acoustic output, smoothness of power output as measure of evenness of excitation of resonant mode of bending wave action, inspection of power output as to frequencies of excited resonant modes including number and distribution or spread of those frequencies, each up to all as useful indicators. Such assessments of viability of locations for transducers constitute method aspects of this invention individually and in combination.
As aid to assessment at least of smoothness of power output, it is further proposed herein to use techniques based on mean square deviation from some reference. Use of the inverse of mean square deviation has the benefit of presenting smoothness for assessment according directly to positive values and/or representations. A suitable reference can be individual to each case considered, say a median-based, such as represented graphically by a smoothed line through actual measured power output over a frequency range of interest. It is significantly helpful to mean square deviation assessment for the reference to have a be normalised standard format; and for the measured acoustic power output to be adjusted to fit that standard format. The standard format may be a graphically straight line, preferably a flat straight line thus corresponding to some particular constant reference value; further preferably the same line or value as found naturally to apply to a distributed mode panel member at higher frequencies where modes and modal action are more or most dense.
In this connection it is seen as noteworthy that whatever function is required for such normalising to a substantially constant reference is effectively also a basis for an equalisation function applicable to input signals to improve lower frequency acoustic output. It is the case that viable distributed mode panel members as such, and with preferential aspect ratios and bending stiffness(es) as in our above patent application, may naturally have acoustic power output characteristics relative to frequency that show progressive droops towards and through lower frequencies where resonant modes and modal action are less densexe2x80x94but, as their frequency distribution as such is usually beneficial to acoustic action in such lower frequency range, such equalisation of input signal can be useful. This lower acoustic power output at lower frequencies is related to free edge vibration of the panel members as such, and consequential greater loss of lower frequency power, greater proportion of which tends to be poorly radiated and/or dissipated, including effectively short-circuited about free adjacent panel edges. As expected, these lower frequency power loss effects are significantly greater for panel members with transducer locations at or near their edges and/or lesser stiffnessesxe2x80x94compared with panel members using in-board transducer locations. However, and separately from any input signal equalisation, significant mitigation of these effects is available by mounting the panel members surrounded by baffles and/or by clamping at the edges of the panel members. Indeed, spaced localised edge clamps can have usefully selectively beneficial effects relative to frequencies with wavelengths greater than the spacing of the localised edge clamps.
Interestingly, for specific panel members of quite high stiffnesses, viable marginal transducer locations include positions having edge-wise correlation with normally in-board locations for transducers arising as preferred by application of teachings or practice such as specifically in our above patent applications. When using transducers in pairs, a first preference was found for marginal transducer locations with said correlation as corresponding to notionally encompassing greatest area. For a substantially rectangular panel member, said correlation can be by way of correspondence with orthogonal or Cartesian co-ordinates, with said first preference represented by associating transducers with diagonally opposite quadrants. However, this was in relation to a particularly high stiffness/high-Q panel member, and is not always true, even for quite (but less) stiff panels, see further below showing promising operation with association in some or adjacent quadrants. For an elliptical panel member said correlation/correspondence can be according to hyperbolic resonant mode related lines as going edge-wards through the in-board locations. Other variously less good, but feasibly viable, pairs of edge locations for transducers were found by investigation based on rotating orthogonal vectors about in-board preferential transducer locations, including close to or at corner positions of panel members. Another inventive aspect regarding corner or near-corner excitation involves suitably mass-loading or clamping substantially at a known in-board optimal or preferential drive location, where it appears that such mass-loaded optimal drive location(s) effectively behave(s) to some useful extent as xe2x80x9cvirtualxe2x80x9d source(s) of bending wave vibrations in the member. This latter may not avoid central intrusion by the mass loading but is clearly germane to successful marginal excitation at corners.
Further investigations have been made, including of panel members having different stiffnesses, specifically again quite high but also much lower and intermediate stiffness panels, in each case of usual substantially rectangular configuration with aspect ratios and axial bending stiffnesses generally as in patent application Ser. No. 08/707,012.
For the higher stiffness panel member, assessment based on smoothness of power output for single transducer locations along longer and shorter edges were generally confirmatory of above preferential co-ordinate positions, i.e. peaking as expected for best locations for a single transducer. However, additionally, longer edges had promising spreads of smoothness measure within about 15% of peak at transducer locations between the co-ordinate positions in each half of the edge and beyond those co-ordinate positions to about one-third length from each corner; and within about 30% along to at least the quarter length positions. For the shorter edges, spreads of smoothness measure were within about 10% between the co-ordinate positions, and within about 25% at quarter length positions. The shorter edges actually showed a better power smoothness measure than the longer edges showed at quarter length positions right through to within about one-tenth length of the corners.
Investigation of combinations of two transducers has also been extended particularly for same and adjacent quadrants with one transducer, for one on each of longer and shorter edges. One transducer can be at one best position along one of the edges for a single transducer, with the other transducer varied along the other edge. For variation along the shorter edge, above preference for one of positions according to co-ordinates of in-board preferential transducer locations is confirmed by best smoothness measure at about six-tenths length. There are also near as good positions at three-quarter length and only a little less good at quarter and third length positions. Moreover, most positions other than below about one-tenth from a corner are better, similar, near as good, or not much worse, than for association with co-ordinates of preferred in-board locations in the same quadrant. For variation along the longer edge, the shorter edge transducer was located at about preferred near six-tenths position, there was then actually marked preference for combinations of transducer locations in adjacent quadrants, with best at just under one-fifth, and slightly better than the 0.42 position at the one-third length position with only a little worse at the one-tenth length position. The quarter length position is actually about the same as for the mid-length position and the adjacent quadrant position of the co-ordinate of preferred in-board location. Self-evidently, these procedures may be continued on an iterative basis, and may then reveal more favourable combinations.
Investigations of much lower stiffness panel members on the basis of smoothness of power output have shown peaking for marginal transducer locations also at about the in-board co-ordinate position, but near as good at quarter length of panel edges, and generally markedly less criticality as to position along the edges in terms of actual achieved modal distribution. This is seen as explicable by interaction between the lower panel stiffness and compliance within the used transducer itself. It appears that the resonant modal distribution of the panel is affected and altered by the transducer location, at least to some extent going with such location. Higher panel stiffnesses substantially avoid such effects. However, such in-transducer compliance and possible interaction with panel stiffness/elasticity is clearly another factor to be taken into account, including exploited usefully.
Investigations of panel members with quite high and much lower stiffnesses clearly reveal rather different cases for application of marginal excitation, including as to more and less criticality as to transducer locations, whether singly or in pairs, and as to less or more interaction with in-transducer compliance. It is thus appropriate to consider a panel member of intermediate stiffness.
For such intermediate stiffness panel member, and much as expected, differences relative to the much lower stiffness panel member include increase in acoustic power output available by edge clamping, markedly increased power for mid-range frequency modes, and stronger modality or peakiness for lower-frequency modes. Tendency towards characteristics of the higher stiffness panel member include stronger preference as best single transducer locations for edge positions on a co-ordinate of optimal in-board transducer locations, also promising feasibility for through the mid-point, but perhaps also at about one-tenth in from corners. For two marginally located transducers, marked preference resulted for the co-ordinate related position of optimal in-board transducer location, with less good but likely viable spread to middle and two-thirds length positions and equality of same quadrant co-ordinate related and two-thirds length positions.
It is evident that differences in materials parameters of panel members beyond basic capability to sustain bending wave action are significant in determining marginal transducer locations; and that use of two or more such transducer locations produces highly individual solutions requiring experimental assessment such as now enabled by teachings hereof.
Also, at least specifically for tested substantially rectangular panel members, it has been found that many if not most, probably going on all, of edge or near-edge locations for transducers that are unpromising as such can be significantly improved (as to bending wave dependent resonant mode distribution and excitement into acoustical response of the member) if associated with localised mass-loading or clamping at one or more selected other marginal position(s) of the panel member concerned. Inventive aspects thus includes association of a said drive position with helpful other mass-loading or clamping position marginal of the panel member.
Regarding use of two or more transducers, exhaustive investigation of combinations of marginal locations is impractical, but teaching is given as to how to find best and other viable marginal locations for a second transducer for any given first transducer marginal location. Indeed, yet further marginal transducer locations could be investigated and assessed according to the teaching hereof. Somewhat likewise, use of localised marginal damping for improving performance for any given transducer marginal location is investigatable and assessable to any extent and number using the teaching hereof, whether for enhancing or reducing contributions of some resonant mode(s), otherwise deliberately interfering with other resonant mode(s), or mainly to increase output power.
It believed to be worthwhile generally to take into account the fact that lowest resonant modes are related to length of the longest natural axis of any panel member, thus that longer edges of substantially rectangular panel members are sensibly always favoured for location of transducers, including doing so wherever feasible at the best position for operation with a single transducer. It is sensible to see this as applying even where use of another transducer is encouraged or intended, again whether for enhancing some resonant mode(s), deliberately interfering with other resonant mode(s) or mainly to increase output power.
Also relevant as a general matter is the fact that the operating frequency range of interest should be made part of assessment of location for transducers, and may well affect best and viable such locations, i.e. could be different for ranges wholly above and extending below such as 500 Hz. Another influencing factor could be presence of an adjacent surface, say behind the panel member at a spacing affecting acoustic performance.
It is inferred or postulated that the nature of preferred said edge or edge-adjacent position(s) tend towards what is fore-shadowed in our above patent application Ser. No. 08/707,012 and other patent applications, typically viewed as affording coupling to more approaching most frequency modes, and doing so more rather than less evenly, perhaps typically avoiding dominance of up to only a few frequency modes. Such suitability may be for lower rather than higher total actual vibrational energy locally in the panel member, but high as to population by frequency modes, i.e. rather than xe2x80x9cdeadxe2x80x9d in the sense of little or no coupling to any or few modes.